According to known technology, multiple testing instruments can be operated to perform testing on multiple components. For example, a manufacturer of multi-layer ceramic capacitors uses a test system to determine the quality of a lot of product before the product is sold to a customer. The test system performs several tests that provide data on the capacitance, dissipation factor and insulation resistance. The data can then be used to sort the parts by tolerance and find those parts that are defective.
Tests are performed in a sequence that varies depending on individual manufacturer requirements. For example the following sequence can be used. Referring to FIGS. 1 and 2, a part can first undergo a capacitance and dissipation factor measurement at one station using a capacitance meter. Referring to FIG. 1 a theoretical plot of voltage across a capacitor being tested versus time is illustrated, where at t0 the part is at zero volts. At t1, the part begins charging. At t2, the part has reached a programmed value. At t3, all measurements are complete and the part can begin discharging. At t4, the part is discharged to zero volts. Referring now to FIG. 2 a theoretical plot of current through the capacitor being tested versus time is illustrated, where at t0 the part is at zero volts, and therefore has no current flowing through the part. At t1, the part begins charging. The part is charged with a constant-current source. At t2, the part is charged so it no longer accepts current. This graph assumes an ideal capacitor and neglects parasitics, such as leakage current. At t3, the part begins discharging, so current flows in the reverse direction until the part reaches zero volts at t4.
The part can then move to another station, where the part can be charged to a programmed voltage by a programmable voltage and current source. The part can then be held at the programmed voltage for a certain period of time, called the “soak time”. After this period of time, an insulation resistance measurement can be performed by a high resistance meter. This measurement returns a single value in units of either current or resistance. The current measured is the leakage current through the capacitor when a voltage is applied, and the resistance is calculated from R=V÷(leakage current) where V is an input parameter.
Referring now to FIG. 3, a theoretical plot of leakage current through the capacitor being tested versus time is illustrated. At t0, the part is at zero volts, so there cannot be any current flow. At t1, the part begins charging. Leakage current values are typically in the picoamps to microamp range, so this measurement must be very sensitive. Therefore, during the charge period, the current (milliamps) is greater than the measurement range, so the output reaches a maximum value. At t2, voltage continues to be applied to the capacitor under test. The leakage current will begin to decrease because the dielectric is becoming more and more polarized. This is due in part to an effect known as dielectric absorption, and the magnitude of the effect will vary with different dielectrics. If the time axis were extended to several minutes or hours, this curve would continue to decrease exponentially until it reached a nominal value. At some point between t2 and t3, the insulation resistance or leakage current measurement is performed. This takes a snapshot of the leakage current at that time. Once that test is completed at t3, the part is discharged. Again, the high discharge current will cause the perceived leakage to be at a maximum in the other direction. At t4, the part returns to zero volts. Once this measurement is complete, the part can be discharged and prepared for sorting based on the values collected or prepared for a repeat test.
As ceramic capacitors become smaller and higher in capacitance, the effects of the dielectric and parasitic elements become more pronounced and more complex. Ideally, the electrical properties of a capacitor would be observed for a long period of time to minimize the effect of parasitics. However, this is not always feasible from a manufacturing stand point because it would take too long to test millions of devices. Therefore, the industry sometimes relies on only a short snapshot of this time to make a determination of the status of the parts. Insuring the accuracy and reliability of the data is crucial, as it directly affects the customers' yield and the quality of the delivered product.
The industry standard for measuring the leakage current through a capacitor is to use an Agilent 4349B High Resistance Meter in combination with a programmable voltage and current source. The Agilent 4349B is a high precision instrument that uses an integrating current-to-voltage converter and a selectable integration time of 10, 30, 100 and 400 milliseconds. The output of the meter is a single current reading after this integration period is complete. The user then relies on this one measurement to decide whether a given capacitor is acceptable or not.
The voltage and current supply used with the meter is a programmable computer-controlled device, such as an Electro Scientific Industries 5412/5422 power supply.
The voltage and current supply and the meter are controlled by a host personal computer that provide commands and data to control both the triggering of the voltage and current supply and the triggering of the meter to measure the resultant data of the tested component, such as the capacitors described. Between these devices, timing between startup charge and the start of measurement must be very well controlled. In addition, multiple supplies and meters are typically operated to test a number of components simultaneously at each of the stations in order to speed up the entire testing process. Thus, timing between each of these combinations of supplies and meters must also be well controlled to minimize variation in testing between the combinations operating at the same time so that variations in results for similar components are not introduced by the testing process, which could obscure the identification of a satisfactory device under test as defective or a defective device under test as satisfactory.